1. Field
The field of the present invention relates to particle beam writing and lithograph technologies for fine image fabrication and, in particular, to a method and system for proximity effect and dose correction for a particle beam writing device.
2. Description of Related Art
In conventional semiconductor manufacturing processes for generating fine images on a plate by lithography, technologies like particle beam writers and an optical projection lithography method that uses a mask consisting of transparent and opaque parts have been used. The plate is coated by a particle beam sensitive resist. An example of a particle beam writer is an electron beam (EB) writer, which is used for writing fine images on both silicon wafers and glass masks for optical projection lithography. A technology that uses electron beam (EB) writers for making fine images for semiconductor integrated circuits directly on the semiconductor wafer is electron beam direct writing (EBDW) technology.
A fundamental problem with conventional lithograph technologies is image quality degradation and resolution limits caused by chemical and physical effects in the process of the technologies. The degradation or difference between obtained image and intended pattern becomes serious with fine images. Proximity effect is one of the most dominant issues among these effects.
In general, proximity effect correction is mandatory for the particle beam writer including electron beam writers. Proximity effect is a degradation or variation of the written image caused by scattering of incidence particles in resist or backscattering or reflecting particles from lower layers of the resist.
Conventional electron beam (EB) writers can write complicated patterns by one shot. Variable Shaped Beam (VSB) type EB writers can write any flexible predefined sized rectangle/triangle/trapezoid by one shot. Cell Projection (CP) type EB writers can write complex patterns defined on a stencil plate by one shot. However, the size of CP cells on the stencil plate has limitations. In general, the allowable size of commonly used CP cells is several microns by several microns.
FIG. 1 depicts an EB writer 100 that can draw patterns by means of both VSB and CP. An electron beam source 102 projects an electron beam 103 that is formed into a rectangular shape 108 with a rectangular aperture 104 formed in a first mask 106 or first shaping aperture and then is projected through a stencil aperture 110b formed in a stencil mask or plate 112. A cell pattern 114 is projected on a wafer surface or substrate 116 from the stencil aperture 110b of the stencil mask 112. As shown in FIG. 1, the stencil mask 112 includes various stencil apertures 110a, 110b, 111a, 111b, for both VSB and CP cell pattern write.
In both VSB and CP type writing, dose or number of charged particles, such as electrons, in a shot can be controlled by exposure time. The quantity of injected particles is called a dose. In general, a higher dose of charged particles provides larger images as a result. FIG. 2A shows a case in which the dose is low, and FIG. 2B shows a case in which the dose is high. As shown, a target pattern or region 200 in FIG. 2A is the same as a target pattern or region 202 in FIG. 2B. However, due to different exposure times, an obtained image 212 of FIG. 2B is larger than an obtained image 210 of FIG. 2A.
Some proximity effect correction methods have been proposed. One method is a dose correction method that corrects proximity effect by controlling dose used for a VSB or CP shot. Another method is proximity effect correction (PEC) by pattern that corrects proximity effect by modifying writing patterns or adding auxiliary patterns. These auxiliary patterns are sometimes referred to as an assist pattern or dummy pattern. PEC methods that use pattern modification, such as slimming down or fatting up some parts of the pattern, and add serif to the pattern for obtaining intended images are also classified as a PEC method that uses auxiliary patterns. Hereafter, these types of correction methods are called Proximity Effect Correction (PEC) by pattern. FIGS. 3A and 3B depict examples of PEC by pattern. FIG. 3A shows a target pattern 300 and an obtained image 302 without PEC by pattern, and FIG. 3B shows a target pattern 310, an auxiliary pattern 312 and an obtained image 314 with PEC by pattern.
Some EB writers for mask writing are implemented with only VSB type functionality. PEC by a pattern method is not suitable for VSB type EB writers because pattern modification or auxiliary patterns increase the number of shots required, and as a result, writing time becomes longer. Therefore, the dose correction method is dominant for VSB type EB writers. On the other hand, by using CP cell type writer, plural patterns are written by one CP shot and dose correction among patterns in the CP cell or to use different dose for writing patterns in the CP cell is generally difficult to achieve.
FIG. 4 depicts a CP cell 400 and illustrates the substantial difficulty of dose collection in the CP cell 400. As shown in FIG. 4, CP cell 400 includes a cell boundary 402 and may comprise a plural patterns 404 written by a single EB shot. As a result of the above mentioned discussion, a problem occurs in which the width of the obtained images vary depending on the position in the CP cell 400, although target width is identical among those patterns.
FIG. 5 depicts the above mentioned case that shows images 404 in the CP cell 400 are fattened or made larger 500 by proximity effect. In general, correction of proximity effect by dose modulation is not achieved between patterns in the CP cell 400 because the images 404 are usually written in one shot.
Computing time is another issue for PEC. With the capability of current computers, more than several hours are required for correction of a layer. A commonly used conventional method is referred to as an Area Density (AD) method, which is an approximation method that uses a grid covering a whole chip area as a means of approximation. As shown in FIG. 6, the chip area 600 is divided into small grids 602 that consist of several micron by micron rectangles. The proximity effect calculation is approximated by Area Density (AD) of each grid, where Area Density is defined by the ratio'of area of patterns in a grid to the area of the grid rectangle. In general, from the original layout pattern 610, the accuracy of the approximation 612 is controlled by adjusting the size of the grid 602 or dense grid-results with higher accuracy. If a coarse grid is selected, computing time will be decreased. A trade-off between accuracy and computing time is possible for this approximation model.
However, the area density (AD) method is not an accurate approximation. Another issue is that the division of a chip by the grid does not fit to the idea of writing by CP cell. As shown in FIG. 7, it is quite inconvenient that the grid for the AD method splits 700 a CP cell to form a grid boundary 702. The reason is that some parts of the CP cell need to be shot by a different dose. However, a CP cell is usually shot only once. To resolve above mentioned issues, an accurate approximation model and method that are applicable to both CP and VSB writing is desired.
In recent trends for manufacturing semiconductor integrated circuits, increasing the number of metal layers for wiring should be considered, and the use of metal materials other than aluminum should also be considered. FIG. 8 depicts a sectional view of a recent semiconductor integrated circuit 800. The top layer 802 of the depicted structure is coated by electron beam sensitive resist 804. Conventionally, the scattering effect caused by reflection or collision of particles or electrons with wiring or via/contact material is not considered enough. Compared with aluminum as a conventionally used material, tungsten (W) 812, tantalum (Ta) and copper (Cu) 810 have high reflective characteristics. In case there is high reflective materials under the resist, particle or e-beam writing requires careful consideration for the proximity effect calculation affected by severe scattering environment.
FIGS. 9A-9B illustrate a calculation result, which reflects how the scattering parameter varies with the change of underlay structure of the resist. FIG. 9A shows an underlay structure of the resist that is assumed in the above calculation. The structure varies from 0 to 20 in terms of number of copper strip. FIG. 9B depicts how the parameter or ratio of reflected energy to incidence energy changes as a result of copper strip number variation. However, the conventional solutions are not practical because procedures for taking underlay structure effect into account are tedious and time consuming. Other aspects of this issue is that combination of vias and wires is diverse in structure and difficult to handle by a computer.
FIGS. 10A-10B show scattering of electrons and a scattering model. FIG. 10A shows a first layered structure 1000 with a Tungsten (W) layer 1002 and a second layered structure 1010 with an Aluminum (Al) layer 1012. Conventional modeling of scattering effect by simple grid 1020 covering semiconductor chip, as shown in FIG. 10B, is not a method that utilizes the predominance of writing by a CP cell. The reason for this weakness is that size and position of exposed regions by CP and VSB writing is not aligned with the model for scattering so that the scattering model is not directly usable for the writing. A more efficient scattering model or calculation method is required.
In general, the scattering model depends on the underlay structure of resist like metal wiring and vias. In other words, parameters given by the model are determined by layout patterns in each layer under the resist. However, it is not suitable to use layout patterns itself for generating the model because the patterns have diverse configurations. A more efficient means for modeling the layout under the resist and quick scattering parameter estimation is required. From a quality control point of view, to understand distribution of dose quantity over the chip area or intensity of dose at any point on the chip is substantially important.